1. Technical Field
Illustrative embodiments described in this patent specification generally relate to a belt driving device to rotate a belt; a fixing device using the belt driving device to convey a recording medium and to apply heat to the recording medium; and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
The fixing device generally includes a rotatable fixing roller or a rotatable fixing belt, each heated, and a rotatable pressure roller or a rotatable pressure belt each pressed against the fixing roller or the fixing belt. A recording medium such as a sheet having an unfixed toner image thereon passes through a nip formed between the fixing roller or the fixing belt and the pressure roller or the pressure belt, so that heat and pressure is applied to the toner image to melt and fix the toner image onto the sheet.
In keeping with recent trends toward reducing energy consumption, a fixing device having a smaller heat capacity is more widely used than a heat storage-type fixing device using a fixing member with a higher heat capacity in which a larger amount of electricity is required. Accordingly, a fixing device having a seamless fixing belt with a smaller heat capacity and a fixing roller including a heat insulating silicone foam or the like has become common in recent years, compared to a fixing device having a fixing roller with a large heat capacity.
The temperature of the fixing belt with a smaller heat capacity can be raised to a target temperature more quickly at startup of the fixing device, thereby reducing warm-up time for the fixing device. Therefore, the fixing belt with a smaller heat capacity is essential for the fixing device to save energy.
As for the construction of the fixing belt, typically a seamless polyimide resin having high heat resistance and strength and low thermal expansion is widely used as a substrate of the fixing belt. A surface layer of the substrate is often coated with an elastic layer formed of silicone rubber or the like, and a fluorinated tube having a superior releasing ability from toner is often further provided as an outermost layer of the fixing belt.
In the fixing device having the above-described configuration, a slight shift in the alignment of multiple rollers around which the fixing belt is wound and the action of the nip formed between the fixing roller and the pressure roller with the fixing belt therebetween can cause axial shifting of the fixing belt. Preventing shifting of the fixing belt is an important problem to be solved for the fixing device.
In particular, growing market demand for an ability to handle various different types of recording media and for greater productivity regardless of the types of the recording media has resulted in a wider nip formed between the fixing roller and the pressure roller. Consequently, shifting of the fixing belt more often occurs at the nip between the fixing roller and the pressure roller, and as a result, the problem of preventing shifting of the fixing belt assumes even greater importance.
To solve the problem, one example fixing device is proposed in which a driving roller and a driven roller are provided with a fixing belt therebetween, and the driven roller is movable in a thrust direction of a rotary shaft thereof. An angle of the rotary shaft of the driven roller relative to the driving roller is changed as the driven roller moves. The above-described fixing device may solve the problem of shifting of the fixing belt caused by twisting of the rollers around which the fixing belt is wound. However, shifting of the fixing belt caused by the action of the nip formed between the fixing roller and the pressure roller with the fixing belt therebetween may not be solved by the above-described fixing device because it is assumed that, in the above-described fixing device, shifting of the fixing belt is corrected only when the movement of the driven roller in the thrust direction is in the opposite direction from the shifting of the fixing belt. Therefore, shifting of the fixing belt caused by factors other than twisting of the rollers cannot be solved by the above-described fixing device.
Another approach is a fixing device having a fixing belt wound around a driving roller and a driven roller swingably provided at substantially the center of the driving roller. The driven roller swings when shifting of the fixing belt occurs to prevent shifting of the fixing belt, taking advantage of the fact that the fixing belt tends to move to the side having greater tension.
However, although shifting of the fixing belt can be prevented by the above-described approach when the fixing belt is elastic such as a rubber belt, a heat-resistant resin belt having a polyimide substrate widely used in recent fixing devices does not have the requisite elasticity. Therefore, shifting of a fixing belt lacking the requisite elasticity cannot be solved by the above-described approach.
Further, because the inner diameter of the fixing belt is the same as the diameter of the rotary shaft of the driven roller, a mechanism in which the driven roller swings when shifting of the fixing belt occurs does not operate as desired, and cannot solve the problem.
Yet another approach is a belt driving device including an endless belt; a driven roller; a shifting detector movable in a direction of a shaft of the driven roller and provided at least one end of the shaft of the driven roller to detect shifting of the endless belt; a support member to support the ends of the shaft of the driven roller such that the driven roller is movable in a direction perpendicular to the shaft thereof; and conversion means for converting displacement of the ends of the shaft of the driven roller into movement in a direction descending from a direction of shifting of the endless belt, that is, a predetermined direction perpendicular to the direction of the shaft of the driven roller, when the shifting detector detects movement of the endless belt to the shaft of the driven roller. The conversion means includes a belt guide member and a guide member having a slope. The endless belt is guided along the belt guide member by a smaller force at a position where the endless belt is displaced to the predetermined direction to prevent shifting of the endless belt.
However, because the endless belt and the belt guide member, and the guide member and the belt guide member of the conversion means, contact each other at one point, respectively, abrasion occurs at the portions where the endless belt and the belt guide member, and the guide member and the belt guide member, contact each other, respectively, in continuing use. Further, because the belt guide member is formed of a resin for the purpose of preventing damage to the endless belt, durability of the belt guide member is a concern.